Researchers have been attempting for the past decade to develop an improved synthetic route to 2-ethyl-2-methylbutanoic acid (EMBA). The problem has been to develop an economically acceptable method for producing the olefin precursor, i.e., 3-methyl-2-pentene (3M2P) and/or 2-ethyl-1-butene (2E1B).
These olefin precursors are useful for the preparation of EMBA. However, the presence of even a few percent of any other C.sub.6 olefin during preparation of this neoacid results in an unacceptable mixture of neoacids and neoacid esters that is exceedingly difficult to separate by distillation.
A number of refinery streams contain 3-methyl-2-pentene, but these streams contain many isomers of hexenes, and are not at all pure enough for the preparation of EMBA. 3-methyl-2-pentene may be produced from codimerizing ethylene with n-butenes over a supported sodium or potassium metal catalyst (see Soviet Union Patent No. 1,145,013 and Great Britain Patent No. 1,142,704), but these products also contain enough other isomers of hexene to make them unsuitable for the preparation of EMBA.
Mixed hexenes are produced as a by-product from the Alphabutol.TM. process (a trademark of the Institut Francais du Petrole), which is used to make 1-butene.
A typical analysis of mixed hexenes discharged from the Alphabutol.TM. process is set forth below:
______________________________________ Component Weight Percent ______________________________________ 3-methyl-1-pentene 26.0 1-hexene 6.5 2-ethyl-1-butene 65.0 Others (C6 + C8) 2.5 ______________________________________
This process is described in the November, 1984, issue of "Hydrocarbon Processing", pp. 118-120. Significantly, the only isohexene contained in the C.sub.6 by-product of the Alphabutol.TM. process is 2-ethyl-1-butene. There are no 2-methylpentenes or 2,3-dimethylbutenes, and essentially no methylcyclopentenes. Although the C.sub.6 by-product of the Alphabutol.TM. process does contain significant amounts of EMBA precursor olefins, the presence of other C.sub.6 olefins results in EMBA product that is not of sufficient purity. Conventional fractional distillation of the mixed hexenes yields streams enriched in desirable hexenes, but these enriched hexene streams still do not yield satisfactory EMBA product.
Processes to recover isobutylene and isoamylene from mixed streams of C.sub.4 or C.sub.5 olefins are known. These involve selectively forming an ether from the isobutylene or isoamylene, separating the ether, and decomposing it back to the C.sub.4 or C.sub.5 iso-olefin plus alcohol. (See U.S. Pat. No. 4,691,073 (Michaelson), which issued on Sep. 1, 1987).
There is only one isomer of isobutylene and only one skeletal isomer of isoamylene. There are four skeletal isomers of isohexene, i.e., 2-methylpentene, 2,3-dimethylbutene, 1-methyl cyclopentene, and 3-methyl-2-pentene (or 2-ethyl-1-butene). All of these will react with an alcohol to form the respective ether.
The present inventors have developed a unique integrated process which combines the Alphabutol.TM. process or like processes with an etherification/decomposition process which recovers a C.sub.6 iso-olefin to produce a 3-methyl-2-pentene product which is substantially free of all other C.sub.6 olefins, except 2-ethyl-1-butene.
The present invention also provides many additional advantages which shall become apparent as described below.